There is an ever increasing need for making black-and-white or coloured copies of two- or three-dimensional patterns. Since, in many cases, copies have to be made very quickly and abundantly, it is an aim to reduce the copy-time and to use materials of less expense. This is one of the main reasons for the fact that electrophotography, as a method of making copies, becomes more popular than normal photography, though the quality of xerographic copies, however, is rather tolerated than welcomed.
Coloured bodies or patterns are usually copied by photographing them. For this purpose, different kinds of films are used, which are expensive and which must be developed by several phases, e.g. rinsing, fixing, washing and drying. A method, which enables to make copies quickly and in an easy way, has been proposed by E. H. Land and H. G. Rogers. It is well known under the trade mark POLARCOLOR. This method is a subtractive three-colour-process, which includes a diffusion of colour molecules into the positive surface, the molecules coming from negative. Although this method is very easy to handle, the film is expensive because of its eight layers.
Another method of making coloured copies has been described in the U.S. Pat. No. 2,297,691, p. 6 of Carlson. This method is well-known as the process of Xerography. It comprises the projection of a colour on a metal plate, whereby the colour had been separated by a filter. In a next step the plate will be developed by fixing coloured ink-dust on it. Then the same process will be repeated for another colour and thus different kinds of ink-dust are deposited in layers. Though this proposal has been made more than 35 years ago, there does not exist any satisfactory working Colour-Xerography up to now.
A further proposal for making coloured copies is made in the French Pat. No. 1,450,843. This patent teaches how to suspend light-absorbing particles of different colours in a fluid, which is placed in between electrodes of certain voltages. When the suspension is exposed to the light, the particles flow to one of the electrodes thus making a visible copy of a pattern. The disadvantage of this method, however, lies in the fact that the particles must change their polarities when exposed to the light.